The present invention relates to a diffractive optical element for use in an optical system and an optical apparatus, and more particularly relates to a diffractive optical element configured so that diffraction gratings made of two materials are in contact with each other.
There is a method for reducing chromatic aberration by a combination of glass materials. On the other hand, other methods are disclosed for reducing chromatic aberration in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Patent Laid-Open No. 4 (1992)-213421, Japanese Patent Laid-Open No. 6 (1994)-324262 and U.S. Pat. No. 5,044,706, which provide a diffractive optical element (hereinafter, this may be called a diffraction grating) having a diffraction function at a lens surface or as a part of an optical system. Such a method utilizes a physical phenomenon that chromatic aberration occurs in opposite directions between at a refractive surface and at a diffractive surface in an optical system with respect to a light ray of a certain reference wavelength.
It is further possible to make such a diffractive optical element function as an aspheric surface by changing the period of its periodic structure, whereby aberration can be reduced remarkably.
In an optical system including a diffractive optical element, if most of light in a using wavelength range is converted to diffracted light of one specific diffraction order (hereinafter called a designed diffraction order or a designed order), the intensity of diffracted light of the other diffraction orders will be low. For instance, the intensity of 0 means the absence of the corresponding diffracted light. However, in the case where the diffracted light of the diffraction orders other than the designed diffraction order exists with some degree of intensity, an image will be formed at a place different from that for the designed diffraction order, resulting in flare light generated in the optical system.
Thus, in order to utilize the chromatic-aberration reducing function of a diffractive optical element, it is necessary that the diffraction efficiency of the diffracted light of the designed diffraction order is sufficiently high over the entire using wavelength range. To this end, it is important to consider sufficiently a spectral distribution of the diffraction efficiency in this designed diffraction order as well as the behavior of the diffracted light of the diffraction orders other than the designed diffraction order.
Herein, the diffraction efficiency of the diffracted light of a certain diffraction order refers to a ratio of the amount of the diffracted light of the diffraction order to the amount of the entire light flux transmitted through the diffractive optical element (this may be called transmittance also).
FIG. 25 shows a diffractive optical element made up of a substrate 109 and a diffraction grating 108 formed on the substrate 109 (hereinafter called a single-layer DOE). D1 is a grating thickness of the diffraction grating 108. FIG. 26 shows the diffraction-efficiency-characteristic curves in a specific and other diffraction orders when this single-layer DOE is formed at a certain surface.
In FIG. 26, the horizontal axis represents a wavelength of the incident light, and the vertical axis represents diffraction efficiency. As described above, a value of the diffraction efficiency represents the ratio of the amount of the diffracted light of each diffraction order to the amount of the entire light flux transmitted through the diffractive optical element. Herein, for the sake of brevity, light reflected by a grating interface is not taken into consideration.
As shown in FIG. 26, the single-layer DOE of FIG. 25 is designed so that the diffraction efficiency in the first diffraction order (bold solid line in the drawing) as the designed diffraction order is the highest in the using wavelength range. The diffraction efficiency in this designed diffraction order becomes the highest at a certain wavelength (hereinafter called a designed wavelength), and is gradually decreased at other wavelengths. The light corresponding to the decrease amount in this designed diffraction order becomes diffracted light of the other diffraction orders, which forms flare light. FIG. 26 shows the diffraction efficiencies in diffraction orders close to the designed diffraction order (zeroth order and second order) also as the other diffraction orders.
Various structures have been proposed for reducing the influences of the thus generated flare light.
As shown in FIG. 27, Japanese Patent Laid-Open No. 9(1997)-127322 has disclosed a diffractive optical element in which three different grating materials 110 to 112 and two different grating thicknesses d1 and d2 are selected optimally, and a plurality of diffraction gratings in close contact with each other are arranged with the same pitch distribution. Hereinafter, a diffractive optical element with such a structure will be called a contacting three-layer DOE. Thereby, as shown in FIG. 28, a high diffraction efficiency can be achieved in the designed diffraction order over the entire visible wavelength range.
A diffractive optical element 113 of FIG. 29, which is disclosed in Japanese Patent Laid-Open No. 2000-98118, is configured so that element portions 114 and 115 each including a diffraction grating are brought closer to each other with an air layer 116 interposed therebetween. Hereinafter, a diffractive optical element with such a structure will be called a stacked (multi-layer) DOE. As shown in FIG. 30A, a high diffraction efficiency can be achieved over the entire visible wavelength range by optimizing the refractive indexes of materials making up each diffraction grating, the dispersion characteristic thereof and the grating thickness of each layer. Additionally, as shown in FIG. 30B, the diffraction efficiencies of the zeroth order diffracted light and the second order diffracted light as unnecessary diffracted light can be generally suppressed (reduced).
A diffractive optical element disclosed in Japanese Patent Laid-Open No. 2004-78166 is the stacked DOE similar to the diffractive optical element disclosed in the above Japanese Patent Laid-Open No. 2000-98118. However, a mixed material of a particulate material and a resin material is used and the grating thickness of each layer is optimized, whereby as shown in FIG. 31A, a still higher diffraction efficiency can be achieved than that of the diffractive optical element of the above Japanese Patent Laid-Open No. 2000-98118. Additionally, as shown in FIG. 31B, the diffraction efficiencies of the zeroth order diffracted light and the second order diffracted light as the unnecessary order diffracted light also can be sufficiently suppressed.
A diffractive optical element 119 of FIG. 32 as disclosed in Japanese Patent Laid-Open No. 2005-107298 and Japanese Patent Laid-Open No. 2003-227913 is configured so that diffraction gratings 117 and 118 made of two different resin materials, respectively, are in close contact with each other at their grating surfaces. Hereinafter, a diffractive optical element with such a structure will be called a contacting two-layer DOE. With this structure, a diffractive optical element at a low cost can be realized, which can be manufactured easily.
According to the contacting three-layer DOE disclosed in the above Japanese Patent Laid-Open No. 9(1997)-127322 and the stacked DOEs disclosed in the above Japanese Patent Laid-Open No. 2000-98118, the diffraction efficiency in the designed diffraction order is 94% or higher over the entire using wavelength range, which is considerably improved as compared with the single-layer DOE. Unnecessary diffracted light causing flare light also can be favorably suppressed to 2% or lower.
However, in the case of an optical system mounted to an optical apparatus such as a still camera and a video camera, even small amount of remaining flare light may be a problem when a high-intensity light source is present in an image-pickup area.
In the stacked DOE disclosed in the above Japanese Patent Laid-Open No. 2004-78166 made of a mixed material of a particulate material and a resin material, the diffraction efficiency in the designed diffraction order is 99.5% or higher and the unnecessary diffracted light is 0.05% or lower over the entire using wavelength range, thus realizing higher performance more than the contacting three-layer DOE disclosed of the above Japanese Patent Laid-Open No. 9(1997)-127322 and the stacked DOEs of the above Japanese Patent Laid-Open No. 2000-98118. Thus, it is expected that flare light will be less noticeable to some extent.
However, a diffractive optical element is still required, which can be manufactured more easily than the stacked DOE disclosed in the above Japanese Patent Laid-Open No. 2004-78166 including an air layer.
Meanwhile, in the case of the contacting two-layer DOE disclosed in the above Japanese Patent Laid-Open No. 2005-107298 and Japanese Patent Laid-Open No. 2003-227913, the performance of this diffractive optical element itself, especially the diffraction efficiency of the first order diffracted light as the designed diffraction order light is about 95 to 97% over the entire using wavelength range, which is not sufficiently high. In other words, the flare light due to the unnecessary diffracted light may cause a problem. Furthermore, since the contacting two-layer DOE disclosed in the above Japanese Patent Laid-Open No. 2005-107298 includes a thick grating as much as about 20 μm or more, there is another problem of the degradation in diffraction efficiency resulting from vignetting of an obliquely incident light ray.